IsComplete of AsynchronousObject returned by BeginInvoke() all true

Question

I'm currently learning about runspaces in Powershell (my end goal is to set up a job scheduling system) to do this I wrote a basic script in order to learn and use runspaces.

What I Expected To Happen:

I expected that when I run the code up to the commented line, this will queue up the 8 jobs and run them within the RunspacePool , running a maximum of 2 at a time.

Running the single line $JobList.AsynchronousObject a few times and should then see more and more IsComplete flags turning from false to true as the jobs complete as they take 20 seconds each due to the Start-Sleep command.

The BeginInvoke command apparently returns an object implementing the IAsycResult interface.

https://learn.microsoft.com/en-us/dotnet/api/system.iasyncresult?redirectedfrom=MSDN&view=netframework-4.8#examples

In the IAsyncResult remarks in mentions polling the IsComplete property to see if an asychronous operation is completed which although not ideal is what I was trying to do below for learning purposes.

Actual:

All the IsComplete flags are true a second after running the top portion of code which is not what I expected

Question:

Does the IsComplete flag represent just whether the script has started executing and maybe that is why they're all true a second after queuing up?

I'm grateful for any assistance or references to further reading anyone is able to provide.

Many Thanks

Nick

#Set up runspace
    $RunspacePool = [runspacefactory]::CreateRunspacePool()
    $RunspacePool.SetMinRunspaces(1)
    $RunspacePool.SetMaxRunspaces(2)

#Create arraylist to hold references to all the instances running jobs
    $JobList = New-Object System.Collections.ArrayList

#Queue up 8 jobs that will take 20 seconds each to complete
#Add the job details to the list so I can poll it's IsComplete property
    $RunspacePool.Open()
    1..8 | ForEach {
        Write-Verbose "Counter: $_" -Verbose
        $PowershellInstance = [powershell]::Create()
        $PowershellInstance.RunspacePool = $RunspacePool
        [void]$PowershellInstance.AddScript({
            Start-Sleep -Seconds 20
            $ThreadID = [appdomain]::GetCurrentThreadId()    
            Write-Verbose "$ThreadID thread completed" -Verbose
        })
        $AsynchronousObject = $PowershellInstance.BeginInvoke()
        $JobList.Add(([PSCustomObject]@{
            Id = $_
            PowerShellInstance = $PowershellInstance
            AsynchronousObject = $AsynchronousObject
        }))
    }
#----------------------------------------------

#List IsComplete should show true as jobs become complete
    $JobList.AsynchronousObject

#Clean up 
    $RunspacePool.Close()
    $RunspacePool.Dispose()

Answer 1

There is no issue. You are forgetting what Asynchronous really means.

When you launch Asynchronous jobs, they don't block the current thread (aka. you're current PowerShell prompt) instead, they create a new thread and run from there. The whole point about Asynchronous jobs is that you can run multiple things at once.

So what happens is that Runspace is created, everything gets set up, Jobs are queued and start to run in new threads, then it keeps going (everything is Async and running in separate threads). It then goes right on to execute the last three lines:

#List IsComplete should show true as jobs become complete
    $JobList.AsynchronousObject

#Clean up 
    $RunspacePool.Close()
    $RunspacePool.Dispose()

Which kills the Runspace and Disposes of it, thereby "completing" the jobs.

If you run everything up to the commented line first. Then start watching $JobList.AsynchronousObject from the PowerShell prompt, then you will see it stepping through the jobs as expected.

Once complete, then you can execute the final two lines to close and dispose of your runspace.

You will have to look at the Job Wait functions if you want to have things wait for you.